In communication systems (both terminal and base station infrastructure), surface acoustic wave (SAW) filters and resonators are widely used. For new standards such as the E-UTRA used in the 5G-TG4-CA, there is growing demand for higher operating frequency, lower insertion loss, higher transmitting power, and/or wider channel bandwidth. New piezoelectric materials or structures are desirable to support the growing demand for higher frequency and wider channel bandwidth. In particular, a high electromechanical coupling coefficient for SAW filters may be desirable.
One of the factors that affect the filter bandwidth is the piezoelectric material's electromechanical coupling coefficient (k2). The higher the electromechanical coupling coefficient, k2, the wider the filter bandwidth may be. Aluminum Nitride (AlN)/diamond multi-layer structures have been suggested may have a very high acoustic wave velocity of about 9500 m/s at AlN thickness of 0.35 wavelength, λ. However, the relatively small k2 of 1.2% may be unsuitable for wideband filter and duplexer application.
Scandium-doped AlN (ScAlN) films have been attempted due to their high piezoelectricity, high thermal conductivity, and relatively high acoustic wave velocity. One such structure is described in “High Q surface acoustic wave resonators in 2-3 GHz range using ScAlN-single crystalline diamond structure” by Hashimoto et al. (Conf.: Ultrasonics Symposium (IUS), 2012 IEEE International), herein expressly incorporated by reference in its entirety.
Similarly, another such structure is described in “Surface acoustic wave propagation characteristics of ScAlN/diamond structure with buried electrode” by Zhang et al. (Piezoelectricity Acoustic Waves and Device Applications (SPAWDA) 2014 Symposium on, pp. 271-274, 2014), herein expressly incorporated by reference in its entirety.
These structures have been explored but have not been adequate to address the specific challenges as described herein.